Continuous ink jet printers comprise a series of electric and fluidic components, including an orifice plate and a charge plate, for generating one or more rows of jets of ink and selectively charging the ink droplets as they form from the jets. Typically, there may be several hundred jets formed in each row, and each jet may be stimulated to produce drops of ink at a given rate. All such drops fall through an electrical deflection field, and those which are charged are deflected into a catcher. Uncharged drops are deposited on a print media positioned below the print head.
One of the critical requirements in ink jet printers is an orifice plate which will produce several hundred jets of ink which are precisely positioned, precisely parallel, and precisely uniform in diameter and shape. The orifice plate must also be compatible with the ink compositions used, and must be resistant to erosion by the ink. In addition, the regions around the orifices should be sufficiently open to provide for cleaning ink and dirt deposits from the orifices for maintaining proper operation.
One method for producing an orifice plate which will produce several hundred jets of ink is described in U.S. Pat. No. 4,184,925. In the '925 patent, the orifice plate is formed of nickel metal, which is resistant to erosion. A photoresist is exposed through suitable masks and developed to produce round photoresist peg areas on each side of the substrate corresponding to the orifices which are to be formed. The orifice plate material, e.g. nickel, is then plated onto the substrate until the orifice material has grown up beyond the height of the pegs. The orifice material then begins to plate inwardly over the edges of the pegs until orifices of the desired size are formed.
Unfortunately, these prior methods placed the photomask in contact with the photo-resist-coated substrate. Dimpled or wavy areas in the substrate or lack of true parallelism caused overexposure, resulting in lack of homogeneity in image shape and dimensions. An additional disadvantage of contact exposure is the sticking and transfer of photoresist particles to the photomask with each exposure, which necessitates frequent cleaning, resulting in premature mask wear. For instance, in formation of resist dots or pegs by the contact exposure method for ink jet orifices, the range of peg diameters typically exceeded two microns variation, whereas the range of dot diameters of the photomask original was only 0.5 microns.
Imaging of photoresist coatings has been done by contacting the surface of coated substrates with a glass mask and exposing with ultraviolet light. For this procedure to yield images having the true dimensions of the photomask, the parts and all the supporting fixtures must be extremely flat in order to avoid shadowing effects where the mask does not locally conform to the substrate. Waviness in the substrate, or bowing, can produce localities where there is not complete contact over large areas.
For imaging that is not highly critical, and to overcome mask sticking, proximity exposure equipment can be used wherein various mechanical schemes are used to establish a planar gap between the substrate and photomask. However, available mask aligner exposure units could not provide satisfactory and reproducible results with the minimum exposure distance being about 0.001 inch. Flat solid foil shims of about 0.0005 inch thickness could be inserted around the mask periphery, but these were impractical to handle because of their fragility.
It is seen then that there is a need for a straightforward method for ensuring close and precise space maintenance between the substrate and the photomask.